Recently, the developments in electronic apparatuses, and the minimization of the size and the weight of such apparatuses have been advancing rapidly. These developments are based on the miniaturization and the enhancement of reliability in semiconductor devices. The application of these semiconductor devices to a semiconductor power device has increased with the increase of the blocking voltage and the current in transistors, and the applications of these transistors to a semiconductor power module which requires the minimization of size and weight are widely conducted.
FIG. 1 shows a circuit diagram of a well-known three-phase inverter bridge using transistors. The portions shown in heavy lines constitute a main circuit portion through which a large current must flow. The reference characters Q1 to Q6 designate power transistors, and the reference characters B1 to B6 designate base input terminals of the power transistors Q1 to Q6. The reference characters P and N designate a positive and a negative DC terminal, respectively. The reference characters U, V, and W designate AC output terminals connected to the terminals of the three-phase AC motor, respectively.
A small capacity device among the semiconductor devices having the circuit construction shown in FIG. 1 is described in the following as a first prior art device:
In this device the wiring is conducted by using a metal conductivity layer printed by gilding or vapor plating on the surface of a print circuit board or an alumina circuit board, thereby resulting in high integration and the minimization or the size and the weight. However, at present the metal conductivity layer is only realized as having a thickness of several microns to several tens of microns, and accordingly, the cross sectional area of the conductivity layer is within a range of 5 .mu.m.sup.2 to 0.1 mm.sup.2 caused by the pattern dimension in practical use, thereby resulting in a maximum allowable limit of 5 to 6 amperes as the current capacity of the wiring.
Accordingly, in a semiconductor power module whose current capacity exceeds 10 amperes, the wiring is conducted as in a manner of the second prior art device shown in FIGS. 2 and 3:
The reference numeral 1 designates a radiating metal base plate, the reference numeral 2 designates an alumina insulating substrate fixed onto the radiating metal base plate 1. The numerals 3a and 3b designate semiconductor power elements. The numerals 5 and 6 designate a base and an emitter input signal terminal. The numerals 7 and 8 designate a positive and a negative DC terminal. The numerals 9a to 9c designate AC terminals. The numerals 10a to 10c designate emitter internal electrodes. The numerals 11a to 11c designate collector internal electrodes. The semiconductor element 3b corresponding to the transistors Q2, Q4, Q6 in FIG. 1 and the emitter internal electrodes 10a to 10c are connected with each other by a fat aluminum wire (300 .mu.m.phi. to 400 .mu.m.phi.) by supersonic wire-bonding, and the semiconductor element 3a corresponding to the transistors Q1, Q3, Q5 in FIG. 1 and the collector internal electrodes 11a to 11c are also connected with each other by a fat aluminum wire 4 by supersonic wire-bonding.
FIG. 3 shows an enlarged view of the construction around the connecting portion of the positive DC terminal 7. The collector internal electrodes 11a to 11c of the semiconductor element 3a corresponding to the transistors Q1, Q3, Q5 in FIG. 1 are soldered to the external electrode of the positive DC terminal 7 through a common metal plate 15.
In such a second prior art semiconductor device, it is required to provide the common metal plate 15 (the one for the DC terminal 8 is not shown) for connecting the collector, emitter internal electrodes 11a to 11c, 10a to 10c of the semiconductor element 3a, 3b with together, respectively, in the wiring of the positive and negative DC terminal 7, 8 through which a large current flows, and furthermore, it is required to connect the common metal plate 15 with the collector, emitter internal electrode 11a to 1c, 10a to 10c by soldering or the like. Therefore, the structure of the semiconductor power device having a medium sized capacity exceeding 10 amperes becomes complicated, and it takes a long time for the wiring in assembling as well as becoming an obstacle to the minimization of the size and the weight.